Aflatoxins are highly carcinogenic substances and can occur as contaminants in food. 1) Since aflatoxin B1 has been shown to be a more potent carcinogen than aflatoxin G1, it is of interest to compare their genetic activity in yeast. 2) The active metabolite of aflatoxin B1 is thought to be an epoxide which can be converted to a dihyrdodial by epoxide hydrase, an enzyme present in liver homogenate. 1,2-epoxy-3,3,3-trichloropropane (TCPO) is an inhibitor of the epoxide hydrase and therefore addition of this inhibitor to the mixture of aflatoxin B1 and liver homogenate should increase the proportion of available epoxide which can react with indicator cells. 3) Yeast cells harvested from log phase cultures grown on medium containing glucose possess cytochrome P-450 while cells harvested from log phase cultures grown on medium containing galactose possess negligible cytochrome P-450. We are comparing the metabolism of aflatoxin B1 to genetically active compounds in cells grown on glucose and in cells grown on galactose.